1. Field of the Invention
The present invention relates generally to a symbol mapping method and apparatus in a mobile communication system, and in particular, to a symbol mapping method and apparatus for a Bit Error Rate (BER)-based M-ary Quadrature Amplitude Modulation (M-QAM) modulation scheme having a cross constellation in a GSM/EDGE Radio Access Network (GERAN) Evolution mobile communication system.
2. Description of the Related Art
Recently, the 3GPP (3rd Generation Partnership Project) TSG-GERAN (GSM/EDGE Radio Access Network) standard conference is proceeding with GERAN Evolution for performance improvement of data transmission rate, spectral efficiency, etc., and is scheduled to adopt the high-order QAM modulation of 16-QAM and 32-QAM as new modulation schemes for improving downlink and uplink performances.
In the GERAN system, different coding schemes can be used based on the Modulation and Coding Scheme (MCS). The modulation schemes used in the GERAN system include Gaussian Minimum Shift Keying (GMSK) and 8-ary Phase Shift Keying (8-PSK). GMSK, a scheme for limiting a bandwidth by passing binary data through a Gaussian Low Pass Filter (LPF) and then performing frequency modulation thereon in a predetermined shift ratio, has a high spectral concentration and high out-band spectral suppression by allowing the data to continuously shift between two frequencies. 8-PSK, a scheme for modulating data such that the data is associated with a phase-shifted code of a carrier, can increase frequency efficiency. As coding schemes used in the GERAN system, there are nine techniques defined for Packet Data Traffic Channels (PDTCHs). The nine techniques include nine schemes of Modulation and Coding Schemes (MCSs) of MCS-1 through MCS-9 for EDGE/EGPRS (Enhanced General Packet Radio Service). In actual communication, a selected one of the various combinations of the modulation schemes and the coding techniques is used. The combinations are identified as MCSs.
FIG. 1 is a diagram illustrating a structure of a downlink transmitter in the GERAN system. Referring to FIG. 1, one Radio Link Control (RLC) packet data block (RLC block) is transferred to a channel encoder 110 where it is coded by a convolutional code, and then provided to an interleaver 120 after undergoing puncturing according to a predetermined puncturing pattern. The data output from the interleaver 120 is transferred to a multiplexer 130 for allocating data on a physical channel. Also, RLC/Medium Access Control (MAC) header information, Uplink State Flag (USF), and code identifier bits are input to the multiplexer 130. The multiplexer 130 partitions the collected data into 4 normal bursts, and allocates each burst to a time slot of a Time Division Multiple Access (TDMA) frame. Data of each burst is modulated by modulator 140 and then input to a training sequence symbol rotator 150. The training sequence symbol rotator 150 adds the Training Sequence Code (TSC) to the modulated data, performs phase rotation on the TSC, and outputs the result to a transmitter 160. For ease of description, a detailed description of an apparatus, for example, a Digital-to-Analog (D/A) converter, additionally needed to transmit the modulated signal will be omitted herein.
FIG. 2 is a diagram illustrating a structure of a receiver in a GERAN system. Referring to FIG. 2, a radio front-end unit 210 receives bursts transmitted in units of time slots via a receive antenna. The received data is input to a buffering and phase derotation unit 220 where it undergoes buffering and phase derotation. The data output from the buffering and phase derotation unit 220 is input to a modulation detection and channel estimation unit 230 that detects a modulation scheme of the received signal and estimates a channel. A training sequence phase derotator 240 detects a degree of phase rotation of the training symbol to detect the modulation scheme, and phase-derotates the training sequence. The data is transferred to a deinterleaver 260 after undergoing equalization and demodulation in an equalizer 250 based on the detected modulation scheme and channel-estimated information. The deinterleaved data is transferred to a channel decoder 270 where the transmitted data is restored.
The foregoing GERAN Evolution system is scheduled to introduce the turbo encoder adopted in 3G Wideband Code Division Multiple Access (WCDMA) as a channel encoder, and to introduce 16-QAM and 32-QAM modulation schemes as a high-order modulation technology. The modulation order is equal to 32/128/ . . . /M-QAM (M=2m+1), which can be expressed with a cross constellation. Determining a cross constellation's size is equivalent to determining a modulation scheme. Accordingly, the GERAN Evolution system can select various modulation schemes according to the channel quality. More specifically, square M-QAM (M=2m+1) can increase the number m of bits per symbol, which is a factor for extending the constellation size, only from m to m+2, for example, like 16/64/256-QAM, but cross M-QAM can increase the number m of bits per symbol even from m to m+1, for example, like 32-QAM or 128-QAM.
Consequently, when the GERAN Evolution system uses a cross modulation scheme such as 32-QAM, there is a need for a method and apparatus for efficiently mapping symbols to bursts in order to improve performance and reliability.